1. Field of the Invention
The present invention relates, in general, to ion chromatography systems for determination of both anionic and cationic analytes and methods for their use.
2. Description of Related Art
Ion chromatography (IC) is a widely used analytical technique for the determination of anionic and cationic analytes in various sample matrices. Ion chromatography today is performed in a number of separation and detection modes. Ion chromatography with suppressed conductivity detection is the most widely practiced form of the technique. In suppressed conductivity detection, an eluent suppression device, termed a suppressor, converts the eluent into a weakly conducting form and enhances the conductance of target analytes. The original suppressors were columns packed with ion-exchange resins in appropriate ionic forms. Those packed-bed suppressors had a relatively large dead volume and required off-line chemical regeneration. To overcome this problem, suppressors based on ion-exchange fibers and other membranes were developed. These suppressors can be continuously regenerated using either acid or base regenerant solutions.
One disadvantage associated with the original membrane suppressors was that an external source of either acid or base regenerant solution typically was used to generate the suppressor continuously. Over the years, various designs of electrolytically-regenerated membrane suppressors have been developed to overcome the limitations associated with the chemically-regenerated membrane suppressors. Exemplars of the electrolytically-regenerated membrane suppressors are disclosed by U.S. Pat. Nos. 4,999,098, 5,248,426, 5,352,360, and 6,325,976, the entire contents of which are incorporated herein by reference for all purposes. Electrolytic suppressors offer several advantages in ion chromatography. They provide continuous and simultaneous suppression of eluents, regeneration of the suppression medium, and sufficient suppression capacity for common ion chromatography (IC) applications. They are easy to operate because the suppressed eluent or water can be used to create regenerant ions electrolytically. Thus, there is no need to prepare regenerant solutions off-line. Also, the suppressors are compatible with gradient separations. They also have very low suppression zone volume, which makes it possible to achieve separations with high chromatographic efficiency.
In ion chromatography, dilute solutions of acids, bases, or salts are commonly used as chromatographic eluents. Traditionally, these eluents are prepared off-line by dilution with reagent-grade chemicals. Off-line preparation of chromatographic eluents can be tedious and prone to operator errors, and often introduces contaminants. For example, dilute NaOH solutions, widely used as eluents in the ion chromatographic separation of anions, are easily contaminated by carbonate. The preparation of carbonate-free NaOH eluents is difficult because carbonate can be introduced as an impurity from the reagents or by adsorption of carbon dioxide from air. The presence of carbonate in NaOH eluents can compromise the performance of an ion chromatographic method, and can cause an undesirable chromatographic baseline drift during the hydroxide gradient and even irreproducible retention times of target analytes. In recent years, several approaches that utilize the electrolysis of water and charge-selective electromigration of ions through ion-exchange media have been investigated by researchers to generate high-purity ion chromatographic eluents. U.S. Pat. Nos. 6,036,921, 6,225,129, 6,316,271, 6,316,270, 6,315,954, and 6,682,701, the entire contents of which are incorporated herein by reference for all purposes, describe electrolytic devices that can be used to generate high purity acid and base solutions by using water as the carrier. Additionally, U.S. Patent Publication Nos. 2003/0132163 and 2008/0173587, incorporated herein by reference for all purposes, describe trap columns that are regenerated electrolytically for removing contaminant ions from eluents and purifying the eluent stream. In one embodiment, the eluent stream flows through a purifying flow channel, including an ion exchange bed. An electric field is applied through the flowing eluent stream in the purifying flow channel, and the contaminant is removed from the eluent stream. Using these devices, high purity, contaminant-free acid or base solutions are automatically generated on-line for use as eluents in chromatographic separations. These devices simplify gradient separations that can now be performed using electrical current gradients with minimal delay instead of using a conventional mechanical gradient pump.
The combined use of the electrolytic eluent generator and suppressor has significantly changed the routine operation of ion chromatographic methods and permits the performance of various ion chromatographic separations using only deionized water as the mobile phase. The use of these electrolytic devices results in significant improvements in the performance of ion chromatography methods by allowing minimal baseline shifts during the gradients, greater retention time reproducibility, lower detection backgrounds, and lower detection limits for target analytes.
There has been a continuing interest in using capillary ion chromatography using separation columns with internal diameters of 1 mm or smaller as an analytical separation tool because of the perceived advantages associated with the miniaturization of separation processes. To date, such systems have not been employed because of the lack of suitable instrumentation and consumables. Typical separation columns in conventional-scale ion chromatography have column internal diameters ranging 2 mm to 9 mm and are operated in flow rate ranging from 0.2 to 5 mL/min.
U.S. Patent Application Publication No. 2006/0057733, the entire content of which is incorporated herein by reference for all purposes, discloses a capillary ion chromatography system using electrolytic generation of potassium hydroxide eluents and suppressed conductivity detection for determination of anions. In this system, the capillary pumping system is used to deliver a stream of deionized water into the capillary KOH eluent generator which consists of a high pressure generation chamber containing a Pt cathode and a low pressure electrolyte reservoir containing a Pt anode. Under the applied electrical field, the potassium ions migrate across the ion exchange connector to combine with hydroxide ions to form a KOH eluent. The concentration of KOH solution formed is proportional to the applied current and inversely proportional to the flow rate of the deionized water carrier stream. Other downstream system components include a degasser unit, an injector, a separation column, a suppressor and a detector.
U.S. Patent Application Publication No. 2006/0057733 further discloses several embodiments of capillary ion chromatography suppressors. In one embodiment, the capillary anion suppressor consists of three chambers. The eluent chamber contains a cation exchange capillary tubing embedded tightly inside a bed of cation exchange resin. Provisions are made so that there are separate fluid connections to the cation exchange capillary tubing in the resin bed. The eluent chamber is physically separated from the cathodic regenerant chamber and anodic regenerant chamber through cation exchange ion exchange membranes. The cathode chamber contains a perforated Pt cathode and the anode chamber contains a perforated Pt anode. Both electrode chambers have two liquid connecting ports (inlet and outlet). In the operation of this type of electrolytic capillary suppressor, the resin bed is continuously regenerated by hydronium ions generated through the electrolysis of water at the device anode. Under the applied electrical field, the hydronium ions generated at the anode of the device migrate across the cation exchange membrane into the cation exchange resin bed. In the meantime, potassium ions exchanged onto the resin bed also migrate across the other cation exchange membrane into the device cathode chamber before going to waste. Water used in electrolysis can be derived from the aqueous effluent from the conductivity detector.
In ion chromatography systems, users need to make a large number of fluid or liquid connections among various system components. To ensure the optimal chromatographic performance, it is critical to ensure that fluid connections are made properly and free of dead volume. For capillary ion chromatography, making proper fluid connections can be very difficult to accomplish because dead volumes as small as several nanoliters can have a dramatically adverse impact on the system performance when the separation flow rates are on the order of several microliters per minute.
In recent years, the use of automated two-dimensional IC methods has gained increasing interest in the determination of analyte ions in environmental samples because those methods provide convenient on-line matrix elimination or diversion and eliminate the needs for cumbersome off-line sample pretreatment steps. In one exemplary two-dimensional IC method, analyte ions are partially resolved from matrix ions on a conventional IC column (e.g., 4-mm ID) in the first dimension, collected onto a capillary concentrator column, then resolved from residual matrix ions on another IC column in the second dimension. The suppressed effluent from the hydroxide eluent in the first dimension is water, which provides the ideal environment for ion-exchange retention and concentration before the transfer to the second dimension. If a 0.4-mm ID (Inner Diameter) capillary IC column is used in the second separation dimension, the column has a one-hundredth cross-sectional area relative to the first dimension column, detection sensitivity is enhanced by a factor of 100. In addition, the two-dimensional IC method makes it possible to combine two different column chemistries. Two-dimensional IC methods with both suppressed conductivity and mass spectrometry detection would provide the advantages of using these methods for determination of parts-per-trillion levels of analyte of interests such as perchlorate and bromate in environmental samples.
Therefore, there are needs to develop capillary ion chromatographs that provide improved means for fluid connections to make capillary ion chromatography a more ease-to-use and reliable analytical technique. In additions, there is also the need to develop multichannel ion chromatographs that offer improved and ease-to-use integration of ion chromatographic separation processes at conventional flow rates and capillary flow rates.
In light of the foregoing, it would be beneficial to have methods and apparatuses which overcome the above and other disadvantages of known ion chromatography systems.